Photoluminescence (PL) spectra of as-made porous Si samples were obtained in a wide peak-wavelength range. After exposure to air or coupling with C60 molecules, the PL peak shifts to a pinning wavelength within the range of 610-630nm. This pinning wavelength is almost independent of the size of the original porous Si nanocrystallites and both redshifting and blueshifting can occur for different sizes. A self-consistent effectivemass calculation shows that the SivO binding states are responsible for the radiation of this pinning wavelength and the blueshift for the large nanocrystallites is due to the additional potential modulation within the Si nanocrystallite by the long-range Coulomb interaction of oxygen ions.